1. Field of the Invention
The present invention relates to a ceramic sheath type thermocouple having a protective tube made of ceramics that can measure high temperatures.
2. Description of the Prior Art
Sheath type thermocouples have been used as protective tubes for metal glow plugs and stainless steel (SUS) sheath type parts used at elevated temperatures and some of them are fabricated from stainless steel. The thermocouples employ a variety of kinds of measuring materials to measure a range of temperature between 300.degree. C. and 1400.degree. C. For example, the stainless steel sheath type thermocouples used in atmosphere at temperatures higher than 1000.degree. C. are made from special heat resisting alloys such as inconel.
Thermocouples that measure the temperature of molten cast iron of about 1500.degree. C. are made by using a wire of Pt--Rh that has a relatively high melting point and is stable in atmosphere and fixing the Pt--Rh wire to a tube of alumina silica fiber. Such thermocouples, however, become unable to take accurate measurements of temperature after about two measurements of the temperature of molten cast iron and thus are normally discarded. The thermocouples are currently not able to be used repetitively a number of times, which necessarily makes them very expensive.
Further, the wires of the thermocouples are often vulnerable to oxidizing or reducing atmospheres and thus generally installed inside a protective tube for use.
Another known ceramic thermocouple for high temperature measurement has a construction in which a W--Re wire is installed in a silicon nitride protective tube which is formed with venting holes at the side and filled with reaction sintered silicon nitride scattered with TiN.
When a conventional protective tube is made of SUS, however, its heat resisting temperature limit during use is as low as 900.degree. C. Further, it is not safe to use the SUS protective tube in a sulfur gas because the metal of the protective tube is eroded by sulfur. When the protective tube is made from a special heat resisting alloy such as inconel, its heat resisting temperature is higher than that of the SUS protective tube but its cost is about two times higher.
The above ceramic sheath type thermocouple has another drawback that when it is used in a high-temperature atmosphere, the W--Re wire will be degraded by oxygen entry into the tube through the venting holes formed in the protective tube. The thermal expansion coefficient of the W--Re wire is 4.8.times.10.sup.-6 /.degree. C., greater than that of a W wire, and stresses caused by the difference in thermal expansion coefficient between the W--Re wire and the material surrounding the wire can deteriorate the wire as the thermocouple is repetitively used.
In the structure of the above ceramic sheath type thermocouple, in which the W--Re wire is contained in the protective tube whose hollow space is filled with reaction-sintered Si.sub.3 N.sub.4 mixed with TiN, however, the responsibility of temperature measurement is not good and needs to be improved.
The thermocouple has the following thermo-electromotive force according to the kind. The thermoelectromotive force of a PR(Pt--Rh) thermocouple is 1.241 mV at 500.degree. C. and 4.833 mV at 1000.degree. C. The thermoelectromotive force of a CA (chromel-alumel) thermocouple is 20.64 mV at 500.degree. C. and 41.269 mV at 1000.degree. C. And the W--Re thermocouple has the thermo-electromotive force of 8.655 mV at 500.degree. C. and 18.257 mV at 1000.degree. C.
The service temperatures of these thermocouples and atmospheres in which they can be used are as shown in FIG. 12. The Pt--Rh thermocouple cannot be used in an inert gas but can only be used in air up to 1500.degree. C. The CA thermocouple can be used in both air and inert gas atmospheres up to 800.degree. C. Further, the W--Re thermocouple is available for use in both air and inert gas atmospheres, up to 400.degree. C. for air and 2300.degree. C. for inert gas.
The PR thermocouple using a Pt--Rh wire has a small thermoelectromotive force, about 1/15 that of the CA thermocouple and about 1/7 that of the W--Re thermocouple, and thus its measuring precision and responsibility are worse than those of the latter thermocouples. For this reason, to measure the temperature of the molten metal in the blast furnace, a worker is required to stay at a measuring location near the smelting furnace for about eight seconds until the temperature stabilizes. Another drawback is that because cast iron adheres to the Pt--Rh wire of the thermocouple when measuring the temperature of the molten metal, a complex process of removing the sticking iron is required. Further, the existing thermocouples can be used for only two or so measurements and the replacement of the thermocouples takes time. Furthermore, the W--Re wire of the thermocouples is easily oxidized in air and thus cannot be used for measuring the temperature of molten cast iron.
Japanese Patent Laid-Open No. 160200/1994 discloses a sheath type thermocouple with hermetically sealing terminals. This thermocouple can prevent measuring errors even when a temperature gradient is produced at the terminal portion by transient temperature changes. A thermocouple wire made of different metal wires-alumel and chromel wires--is installed in a stainless steel sheath with an inorganic insulating material filled in the sheath to isolate the sheath and the thermocouple wire and with the base side of the sheath hermetically sealed with the hermetically sealing terminal portion. A ceramic end plate of the hermetically sealing terminal portion is attached with two kovar piercing pipes, through which insulating sleeves are inserted. The thermocouple wires pass through the insulating sleeves and are drawn out without directly contacting the piercing pipes. Where a stainless steel sheath is used as a protective tube, if gases such as sulfur are present in the atmosphere, the stainless steel sheath which is directly exposed to an external atmosphere will be corroded, degrading the durability of the thermocouple.